(1) Field of the Invention
The present invention relates to a color image forming apparatus such as a color printer, etc., and more detailedly, relates to a tandem type color image forming apparatus wherein a multiple number of photoreceptors are charged so as to develop color images by developing devices holding different color toners.
(2) Description of the Prior Art
Recently, in the field of color electrophotographic processing, tandem type color image forming apparatuses in which a multiple number of photoreceptor drums are arranged in line to obtain a color image of multiple colors of toner have been used in order to enhance the printing speed. This tandem type color image processing lends itself to color image forming apparatuses and multi-color image forming apparatuses for outputting image formed articles of reproduction and composition of color images and multi-color images by successively transferring a plurality of color separation images for color image data or multi-color image data, in a layered manner, as well as machines having these functions.
FIG. 1 is a front sectional view showing the overall configuration of a digital color copier 1 as a typical example of the tandem type. Copier body 1 has an original table 111 and an aftermentioned control panel on the top thereof and has an image reading portion 110 and an image forming unit 210 within. A reversing automatic document feeder (RADF) 112 is arranged on the top surface of original table 111 in a predetermined position with resect to the original table 111 surface whilst being supported so as to be opened and closed relative to original table 111.
RADF 112, first, conveys an original so that one side of the original opposes image reading portion 110 at the predetermined position on original table 111. After the image scanning of this side is completed, the original is inverted and conveyed to original table 111 so that the other side opposes image reading portion 110 at the predetermined position on original table 111. Then, when RADF 112 completes image scanning of both sides of one original, the original is discharged and the duplex copy conveying operation for a next document is implemented. The operation of the conveyance and face inversion of the original is controlled in association with the whole copier operation.
Image reading portion 110 is disposed below original table 111 in order to read the image of the original conveyed onto original table 111 by means of RADF 112. Image reading portion 110 includes original scanning portion 113 and 114 which reciprocates along, and in parallel to, the undersurface of original table 111, an optical lens 115 and a CCD line sensor 116 as a photoelectric converting device.
This original scanning portion 113 and 114 is composed of first and second scanner units 113 and 114. First scanner unit 113 has an exposure lamp for illuminating the original image surface and a first mirror for deflecting the reflection image of light from the original toward the predetermined direction and moves at the predetermined speed in a reciprocating manner in parallel with, whilst being kept a certain distance away from, the undersurface of original table 111. Second scanner unit 114 has second and third mirrors which deflect the reflected light image from the original, deflected by first mirror of first scanner unit 113 toward the predetermined direction and moves in a reciprocating manner at a speed related to that of first scanner unit 113 and in parallel thereto.
Optical lens 115 reduces the reflected light image from the original, thus deflected by third mirror of the second scanner unit, so that the reduced light image will be focused on the predetermined position on CCD line sensor 116.
CCD line sensor 116 implements sequential photoelectric conversion of the focused light image into electric signals and outputs them. CCD line sensor 116 is a three-line color CCD which reads monochrome or color images and outputs line data as to color separation components R (red), G (green) and B (blue). The original image information thus obtained in the electric signal form from this CCD line sensor 116 is further transferred to the image processor where predetermined image data processes are performed.
Next, the configuration of image forming unit 210 and the configuration of the components related to image forming unit 210 will be described.
Provided below image forming unit 210 is a paper feeding mechanism 211 which separates a sheet of paper (recording medium) P, one by one, from a stack of paper held in a paper tray and feeds it toward image forming unit 210. The paper P thus separated is delivered into image forming unit 210 with its timing controlled by a pair of registration rollers 212 located before image forming unit 210. The paper P with an image formed on its one side is conveyed and re-fed to image forming unit 210 in time with image forming of image forming unit 210.
Arranged under image forming unit 210 is a conveyer and transfer belt mechanism 213. A conveyer and transfer belt 216 of conveyer and transfer belt mechanism 213 is wound and tensioned between a driving roller 214 and an idle roller 215 so that the upper and lower parts of the belt extend approximately parallel to each other. The conveyer and transfer belt 216 electrostatically attracts paper P to itself to convey it. Further, a pattern image density measuring unit is provided under and in proximity to conveyer and transfer belt 216.
Arranged in the paper conveyance path, downstream of conveyer and transfer belt mechanism 213 is a fixing unit 217. This fixing unit 217 fixes the transferred toner image onto paper P.
The paper P having passed through the nip between a pair of fixing rollers of fixing unit 217 passes through a conveyance direction switching gate 218 and is discharged by discharge rollers 219 to a paper output tray 220 attached to the outer wall of copier body 1.
This switching gate 218 selectively connects the conveyance path of paper P after fixing with either the path to discharge paper P to the outside of copier body 1 or the path to recirculate paper P toward image forming unit 210. The paper P which is designated to be conveyed again to image forming unit 210 by means of switching gate 218 is inverted by means of a switch-back conveyance path 221 and then re-fed to image forming unit 210.
Arranged above, and in proximity to, conveyer and transfer belt 216 in image forming unit 210 are the first image forming station Pa, the second image forming station Pb, the third image forming station Pc and the fourth image forming station Pd, in the above mentioned order from the upstream side of the paper conveyance path.
Conveyer and transfer belt 216 is frictionally driven by driving roller 214 in the direction indicated by arrow Z in FIG. 1, and carries paper P which is fed by paper feeding mechanism 211 as stated above and sequentially conveys it through image forming stations Pa to Pd.
All the image forming stations Pa to Pd are of a substantially identical configuration. Each image forming station Pa, Pb, Pc and Pd has a photoreceptor drum 222a, 222b, 222c and 222d, which is driven in the rotational direction indicated by arrow F in FIG. 1.
Provided around each photoreceptor drum 222a-222d, are a primary charger 223a, 223b, 223c and 223d for uniformly charging photoreceptor drum 222a-222d, a developing unit 224a, 224b, 224c and 224d for developing the static latent image formed on photoreceptor drum 222a-222d, a transfer charger 225a, 225b, 225c and 225d for transferring the developed toner image on photoreceptor drum 222a-222d to paper P, cleaning unit 226a, 226b, 226c and 226d for removing the leftover toner from photoreceptor drum 222a-222d, in this order with respect to the rotational direction of each photoreceptor drum 222a-222d. 
Arranged above photoreceptor drums 222a-222d are laser beam scanner units 227a, 227b, 227c and 227d, respectively. Each laser beam scanner unit 227a-227d includes: a semiconductor laser element (not shown) for emitting a spot beam modulated in accordance with the image data; a polygon mirror (deflecting device) 240 for deflecting the laser beam from the semiconductor laser element, in the main scan direction; an f-theta lens 241 for focusing the laser beam deflected by polygon mirror 240 onto the surface of photoreceptor drum 222a-222d; and mirrors 242 and 243.
The pixel signal corresponding to the black component image of a color original image is supplied to laser beam scanner unit 227a; the pixel signal corresponding to the cyan color component image of a color original image is supplied to laser beam scanner unit 227b; the pixel signal corresponding to the magenta color component image of a color original image is supplied to laser beam scanner unit 227c; and the pixel signal corresponding to the yellow color component image of a color original image is supplied to laser beam scanner unit 227d. 
In this arrangement, the static latent images corresponding to the color separations of the original image information are formed on photoreceptor drums 222a to 222d. Developing units 224a, 224b, 224c and 224d hold black toner, cyan color toner, magenta color toner and yellow color toner, respectively. The static latent image on photoreceptor drum 222a-222d is developed by the toner of a corresponding color. Thus, the color separations of the original image information are reproduced in image forming unit 210 as toner images of different colors.
Provided between the first image forming station Pa and paper feeding mechanism 211 is a paper-attraction charger 228, which electrifies the conveyer and transfer belt 216 surface so that paper P fed from paper feeding mechanism 211 can be conveyed without any slip or slide, whilst being reliably attracted to conveyer and transfer belt 216, from the first image forming station Pa to the fourth image forming station Pd.
An erasing device 229 is arranged approximately right above driving roller 214 located between the fourth image forming station Pd and fixing unit 217. Applied to this erasing device 229 is an alternating current for separating paper P electrostatically attracted to conveyer and transfer belt 216, from the belt.
In the thus configured digital color copier, cut-sheet type paper is used as paper P. When paper P is delivered from the paper feed cassette into the guide along the paper conveyance path of paper feeding mechanism 211, the leading edge of paper P is detected by a sensor (not shown), which outputs a detection signal, and based on the detection signal the paper is briefly stopped by a pair of registration rollers 212.
Then, paper P is sent out in synchronization with image forming stations Pa to Pd, onto conveyer and transfer belt 216 that is rotating in the direction of arrow Z in FIG. 1. At this point, conveyer and transfer belt 216 has been charged in a predetermined manner by paper attraction charger 228 as stated above, so that paper P is stably fed and conveyed during its passage through all the image forming stations Pa to Pd.
In each image forming station Pa-Pd, the toner image of each color is formed so that the different color images are superimposed on the support surface of paper P which is conveyed whilst being electrostatically attracted by conveyer and transfer belt 216. When transfer of the image formed by the fourth image forming station Pd is completed, paper P is separated by virtue of the erasing charger, continuously starting at its leading edge, from conveyer and transfer belt 216 and introduced into fixing unit 217. Finally, paper P having the toner image fixed thereon is discharged through the paper discharge port (not shown) onto paper output tray 220.
In the above description, the photoreceptors are exposed to scanning laser beams from laser beam scanner units 227a-227d, so that optical images are written onto the photoreceptors. However, instead of the laser beam scanner units, another optical writing system (LED head) made up of a light emitting diode array with a focusing lens array may be used. In this case, an LED head is smaller in size compared to the laser beam scanner unit and has no moving parts hence is silent. Therefore, this LED head can be preferably used for an image forming apparatus, such as a tandem type digital color copier, which needs multiple optical writing units.
In actual usage circumstances, such a color image forming apparatus is not only used for color printing but is often used for printing of monochrome (black and white) images. A typical operational control made in accordance with user mode selection will be described with reference to the flowchart shown in FIG. 2. First, when color image output mode is selected (Y at Step S1), all the photoreceptors 222a, 222b, 222c and 222d are set at the ordinary positions where they come in contact with conveyer and transfer belt 216 (S2). Then all the photoreceptors 222a, 222b, 222c and 222d are driven to rotate to implement charging, development and other necessary operations for each of the photoreceptors 222a, 222b, 222c and 222d, in accordance with the electrophotographic process (S3), whereby a color image is formed on a sheet of paper.
On the other hand, when black/white image output mode is selected (N at S1), a separation/abutment mechanism is actuated so that photoreceptors 222b, 222c and 222d for yellow (Y), magenta (M) and cyan (C) are separated from conveyer and transfer belt 216 (S5). Then, drives of these photoreceptors 222b, 222c and 222d are turned off to stop them rotating (S6). At the same time, charging, development and other necessary operations for these photoreceptor 222b, 222c and 222d are turned off (S7). In this condition, photoreceptor 222a for black development is driven to rotate (S8) to implement charging, development and other necessary operations for the photoreceptor 222a for black development, in accordance with the electrophotographic process (S9) to thereby produce a monochrome image with black toner on a sheet of paper.
Conventionally, when the black/white image output mode is selected, photoreceptors 222b, 222c and 222d, other than photoreceptor 222a for black development, are set into a non-active state by stopping the rotation or some other way and caused to part with transfer and conveyance belt 216. Accordingly, no surface coatings of photoreceptors 222b, 222c and 222d unused in the black/white image output mode will be abraded by the cleaning blades and other components or by printing paper, transfer and conveyance belt 216, etc.
Usually, the image forming apparatus of this kind is used more often for monochrome printing than for color printing, hence there is a drawback that the photoreceptor for black images becomes worn away relatively faster than the photoreceptors for other colors. As a result, the four photoreceptors for the four colors of toners Y, M, C and black, differ in amount of abrasion, hence the ways of degradation of the photoreceptors differ between the toner colors. If the photoreceptors are abraded and degraded differently from one color to another, there will occur color imbalance in color image as the number of copies increases.
In this case, since the degradation rates of the drums differ between color types of developing devices, even if only one of them degrades, all the drums should be replaced. Otherwise, color imbalance between the new drum and the other drums which have not been replaced, occurs, resulting in failure to obtain good image quality. In other words, the interval of drum replacement is determined by the most intensively degraded drum among the four, i.e., the drum for black development. This results in being wasteful and uneconomical.
As countermeasures, Japanese Patent Application. Laid-open Hei 10 No.10-333393, Japanese Patent Application Laid-open Hei 11 No.24358 and Japanese Patent Application Laid-open Hei 11 No.52599, disclose configurations in which an xcex1-Si or xcex1-SiC photoreceptor is used for that for black development so as to enhance the photoreceptor life while OPCs(organic photoreceptors) are used for those other than that for black development.
There is, however, a problem that xcex1-Si and xcex1-SiC photoreceptors are less chargeable. As a solution to this drawback, Japanese Patent Application Laid-open Hei 10 No.10-333393 specifies the thickness of the photoconductive layer to be 30 xcexcm or more and its difference in surface potential from the other organic photoreceptors to be equal to or lower than 200 V. Japanese Patent Application Laid-open Hei 11 No.24358 proposes that the applied voltage to the xcex1-Si photoreceptor should be 1.05 to 2.50 times the application voltage to the organic photoreceptors. Further, Japanese Patent Application Laid-open Hei 11 No.52599 is aimed at increasing the chargeability by adding an xcex1-SiC surface layer.
In the above way, in order to extend the life of the photoreceptor for black development while making up for the low chargeability of the xcex1-Si or xcex1-SiC photoreceptor, it is necessary to make complicated charge control for black development, resulting in the need of extra cost. Further, since, other than the charge control, there are differences in light sensitivity and susceptivity to temperature/humidity, between the xcex1-Si or xcex1-SiC photoreceptor and the organic photoreceptor, light exposure, transfer conditions and other factors differ between the xcex1-Si or xcex1-SiC photoreceptor for black development and the organic photoreceptors for development other than black. Therefore, a different control method of the photoreceptor for black development from that for the photoreceptors for the other colors should be used, thus again resulting in the need of extra cost.
The xcex1-Si or xcex1-SiC photoreceptors disclosed in Japanese Patent Application Laid-open Hei 10 No.10-333393, Japanese Patent Application Laid-open Hei 11 No.24358 and Japanese Patent Application Laid-open Hei 11 No.52599, have the problem that their production cost is obviously high compared to the organic photoreceptors. Further, as another problem, they consume large amounts of black toner, as is well known.
As the countermeasures against the above problems, Japanese Patent Application Laid-open 2000 Nos. 242056 and 242057 propose configurations where the drum for black development alone is increased in diameter or increased in film thickness. Japanese Patent Application Laid-open 2001 No.51467 refers to use of a non-contact type charging means only for black development, increase in film thickness and use of a resin having a large viscosity-average molecular weight. Further Japanese Patent Application Laid-open 2000 No.330303 discloses a polycarbonate copolymer resin as the resin for tandem photoreceptors.
Further, provision of a protective layer on only the photoreceptor for black development has been also investigated as an optional method.
Use of a different photoreceptor only for black development in the above ways increases the management tasks. Further, use of a resin having a large viscosity-average molecular weight makes it difficult to apply it.
The solution of increasing the coating film thickness entails the problems that the amount of electrification of the photoreceptor decreases and that the resolution decreases, and other problems. Enlargement of the drum diameter makes the apparatus bulky. Almost all the photoreceptors used at present have no protective coating. This implies that an effective protective coating has not yet been developed.
Moreover, drum wear is attributed to abrasion with printing paper, cleaning blade, charger, developing portion and others, and among these the main cause is considered to be abrasion with the cleaning blade. Therefore, if a non-contact type charging means were used for the charging means for black development only, this will not be a principal, or valid solution, to the drum wear though it is better than nothing.
On the other hand, Japanese Patent Application Laid-open Hei 5 No.53414 and Japanese Patent Application Laid-open Hei 11 No.249452 refer to tandem type image forming apparatuses involving a cleanerless system. These publications, however, are aimed solely at making the machines compact and have no provisions for extending the photoreceptor life. Japanese Patent Application Laid-open Hei 8 No.106197 discloses an image forming apparatus of a multi layer transfer system wherein the amount of charge or volume resistivity of the toner is varied, step by step, one from another, from the preceding development and transfer process to the latter development and transfer process. This publication, however, is aimed at improvement of transfer performance for OHP sheets and involves no reference to the amount of photoreceptor wear and realization of simultaneous maintenance of the photoreceptors, which are the subject matter of the present invention.
Japanese Patent Application Laid-open Hei 9 No.319179 refers to a color image forming apparatus in which the amount of toner adherence at the preceding transfer step and that at the latter transfer step are controlled. This publication, however, is aimed at improvement of image quality against the reverse toner transfer problem and involves no reference to abrading force against the photoreceptor (or the speed of abrading the photoreceptor) and realization of simultaneous maintenance of the photoreceptors, which are the subject matter of the present invention.
It is therefore an object of the present invention to provide a color image forming apparatus which can solve the conventional problems stated above and enables the photoreceptors and toners for all colors to be used and have the same durability, needing a lower cost.
The inventors hereof have eagerly studied in view of the problems in the prior art and demands and found that the life of the photoreceptor for black development can be extended by keeping the abrading force (the speed at which the photoreceptor surface is worn away) of the black toner or the carrier used therewith if a dual-component developer is used, against the photoreceptor surface (coating film), lower than the abrading force of the other developing toners or their carriers, against the photoreceptor surfaces, and have successfully achieved the present invention. Accordingly, it becomes possible to make the speed at which each drum is worn away equal to that of the others, hence it is possible to avoid one drum alone being degraded in an early stage. As a result, it is possible to avoid the problem of failing to obtain good image quality due to a color imbalance between the drums, which would occur when only a single drum was replaced instead of replacing all the drums as used to be done in the conventional configuration. That is, the intervals for replacement of all the four drums are made equal, thus making it possible to avoid wasteful replacement.
A method for making the photoreceptor for black development different in the amount of abrasion from the photoreceptors for other developing colors can be achieved by controlling the abrading force of the toner against the photoreceptor surface, or by differentiating the indirect factors, i.e., the abrading force depending on the toner surface treating agent, the fluidity of the toner and/or the adhering amount of the toner onto the photoreceptor, between the black toner and the other developing toners. Another method can be achieved by making a distinction between the developing toners by making the developing toners different in hardness or using different binders having different hardnesses. Further, when dual-component developers are used, the amount of abrasion can be controlled by differentiating the abrading force of the carrier used with the toner against the photoreceptor coating and/or the adhered amount of the toner onto the photoreceptor, between the toner for black development and the other developing toners.
In the image forming apparatus according to the present invention, since it is not necessary to make the photoreceptor for black development different from the other developing photoreceptors, all the photoreceptors can be configured in a common configuration, which provides improved maintenance performance and a cost advantage.
To sum up, the image forming apparatus of the present invention is provided in the forms of the following configurations and structures.
(1) An image forming apparatus having a tandem system configuration, including: a multiple number of photoreceptors which each form an electrostatic latent image; and a multiple number of developing devices each holding a different color toner from the others for development of the corresponding static latent image, characterized in that the plural toners at least include a black toner, and the abrading force of the black toner against the photoreceptor surface is adjusted so as to be smaller than the abrading force of the other developing toners.
(2) The image forming apparatus defined in (1) above, characterized in that the multiple toners are configured so that each toner is externally added with a surface treating agent on the surface thereof and the abrading force of the black toner will be smaller than the abrading force of the other developing toners.
(3) The image forming apparatus defined in (2) above, characterized in that the amount of the surface treating agent added to each of the other developing toners is specified to be greater than the amount of the surface treating agent added to the black toner.
(4) The image forming apparatus defined in (2) or (3) above, characterized in that the surface treating agent consists of at least one or more kinds of additives, and the secondary particle size of the additive which is added in the black toner and presents the largest secondary particle size is smaller than the secondary particle size of the additive which is added in each of the other developing toners and presents the largest secondary particle size.
(5) The image forming apparatus defined in (4) as above, characterized in that the primary particle of the additive which is added in the black toner and presents the largest secondary particle size has a more rounded shape than the primary particle of the additive which is added in each of the other developing toners and presents the largest secondary particle size.
(6) The image forming apparatus defined in any one of (2) to (5) above, characterized in that A less than B holds where A represents the remaining ratio of the surface treating agent added in the black toner and B represents the remaining ratio of the surface treating agent added in the other developing toners.
(7) The image forming apparatus defined in any one of (2) to (6) above, characterized in that the surface treating agent added in the black toner consists of silica only while the surface treating agent added in the other developing toners consists of at least one of silica, titanium oxide, alumina and white organic fine particles.
(8) The image forming apparatus defined in (1) above, characterized in that the fluidity of the black toner is higher than the fluidity of the other developing toners.
(9) The image forming apparatus defined in (8) above, characterized in that the apparent density (AD) of the black toner is greater than the apparent density (AD) of the other developing toners.
(10) The image forming apparatus defined in (8) or (9) above, characterized in that the decay index (HB) of the black toner is smaller than the decay index (HC) of the other developing toners, where the decay index (H) is defined as the number of applications of tapping vibration to the compacted toner which was compressed while tapping, until the compressed toner decays.
(11) The image forming apparatus defined in (8) above, characterized in that the total added amount of wax in the black toner is lower than the total added amount of wax in each of the other developing toners.
(12) The image forming apparatus defined in (11) above, characterized in that the lowest peak temperature among the DSC peak temperatures of the wax in the black toner is higher than the lowest peak temperature among the DSC peak temperatures of the wax in the other developing toners.
(13) The image forming apparatus defined in (1) above, characterized in that, with the toners, the amount of the black toner adhering to the photoreceptor is lower than the amount of each of the other developing toners to the corresponding photoreceptor.
(14) The image forming apparatus defined in (13) above, characterized in that the absolute value of the amount of charge on the black toner is higher than the absolute value of the amount of charge on the other developing toners.
(15) The image forming apparatus defined in (13) or (14) above, characterized in that the volume resistivity of the black toner is higher than the volume resistivity of the other developing toners.
(16) The image forming apparatus defined in (1) above, characterized in that the binder resin contained in the black toner and the binder resin containers in the other developing toners are different in physical properties or type.
(17) The image forming apparatus defined in (16) above, characterized in that the durometer hardness of the black toner at normal temperature is lower than the durometer hardnesses of the other developing toners at normal temperature.
(18) The image forming apparatus defined in (17) above, characterized in that the durometer hardness of the binder resin contained in the black toner, at normal temperature is lower than the durometer hardness of the binder resin contained in the other developing toners, at normal temperature.
(19) The image forming apparatus defined in (18) above, characterized in that the durometer hardness of the binder resin contained in the black toner, at normal temperature is smaller by 10 or more in the durometer scale than the durometer hardness of the binder resin contained in the other developing toners, at normal temperature.
(20) The image forming apparatus defined in any one of (16) to (19) above, characterized in that the binder resin contained in the black toner is the same kind as the binder resin contained in the other developing toners, and the binder resin contained in the black toner has a smaller weight average molecular weight than the binder resin contained in the other developing toners.
(21) The image forming apparatus defined in (16) above, characterized in that the binder resin contained in the black toner is the same kind as the binder resin contained in the other developing toners, and the peak or shoulder of the binder resin contained in the black toner, which is located on the highest molecular weight side in the molecular weight distribution of the THF (tetrahydrofuran) solubles of the binder resin by GPC, exists at a position to the lower molecular weight side than the peak or shoulder of the binder resin contained in the other developing toners, which is located on the highest molecular weight side.
(22) The image forming apparatus defined in (16) above, characterized in that the binder resin contained in the black toner is the same kind as the binder resin contained in the other developing toners, and the THF insolubles of the binder resin contained in the black toner is smaller in quantity than the THF insolubles of the binder resin contained in the other developing toners.
(23) The image forming apparatus defined in (16) above, characterized in that the binder resin contained in the black toner is a non-cross-linked type resin while the binder resin contained in the other developing toners is a cross-linked type resin.
(24) The image forming apparatus defined in any one of (1) to (23) above, characterized in that each of the plural toners is used with a carrier so as to constitute a dual-component developer.
(25) The image forming apparatus defined in (24) above, characterized in that the black toner concentration upon development is lower than the concentration of the other developing toners.
(26) The image forming apparatus defined in (25) above, characterized in that the concentration of the black toner is lower by 0.5 to 2.0% than the concentration of the other developing toners.
(27) The image forming apparatus defined in (25) above, characterized in that the concentration of each of the multiple toners upon development falls within the range of 3 to 6%.
(28) An image forming apparatus having a tandem system configuration, including: a multiple number of photoreceptors which each form an electrostatic latent image; and a multiple number of developing devices each holding a different color toner from the others for development of the corresponding static latent image, characterized in that the plural toners at least include a black toner, and the abrading force of the carrier used with the black toner against the photoreceptor surface is adjusted so as to be smaller than the abrading force of the carrier used with the other developing toners against the photoreceptor surface.
(29) The image forming apparatus defined in (28) above, characterized in that the particle size of the carrier used with the black toner is smaller than the particle size of the carrier used with the other developing toners.
(30) The image forming apparatus defined in (29) above, characterized in that the mean particle size of the carrier used with the black toner is smaller by the range of 5 to 15 xcexcm than the mean particle size of the carrier used with the other developing toners.
(31) The image forming apparatus defined in (28) above, characterized in that the particle size of the carrier used with each of the multiple toners falls within the range of 60 to 110 xcexcm.
(32) The image forming apparatus defined in (28) above, characterized in that the saturation magnetization of the carrier used with the black toner is lower than the carrier used with the other developing toners.
(33) The image forming apparatus defined in (32) above, characterized in that the saturation magnetization of the carrier used with the black toner is lower by the range of 5 to 20 emu/g than the carrier used with the other developing toners.
(34) The image forming apparatus defined in (28) or (32) above, characterized in that the shape of the carrier used with the black toner is higher in sphericity than the shape of the carrier used with the other developing toners.
(35) The image forming apparatus defined in (28), (32) or (34) above, characterized in that the carriers used with the multiple toners all have the same or almost the same means particle size falling within a permissible range of xc2x10.5 xcexcm, and the specific surface area of the carrier used with the black toner is smaller than the specific surface area of the carrier used in the other developing toners.
(36) The image forming apparatus defined in (28), (32), (34) or (35) above, characterized in that the current value of the carrier used with the black toner is lower than the current value of the carrier used with the other developing toners.
(37) The image forming apparatus defined in (36) above, characterized in that the current value of the carrier used with the black toner is lower by the range of 50 to 100 xcexcA than the current value of the carrier used with the other developing toners.
(38) The image forming apparatus defined in (28), (32), (34), (35) or (36) above, characterized in that the carriers is composed of resin-coated cores, one or more kinds of cores, selected from iron powder, ferrite and magnetite, and the carrier used with the black toner has a different type of core from that of the carrier used with the other developing toner.
(39) A developer used in the image forming apparatus defined in (1), (2), (8), (16), (24), (28), (32), (34), (35), (36) or (38) above, characterized in that.